The present invention relates to integrated circuit liquid crystal hot spot testing. More particularly, the invention relates to a method for identifying failure sites on an integrated circuit chip by detecting hot spots.
In semiconductor fabrication, failure analysis is a crucial step to ensuring high yields of semiconductor devices from the manufacturing process. When an integrated circuit (IC) chip fails in service or testing, the nature and cause of the failure must be determined in order to prevent a recurrence of such a failure in the same or similar products.
An IC chip is typically built on a silicon substrate with multiple layers of insulating and conducting materials. Integrated circuit chips designed by Xilinx, Inc., assignee of the present invention, have six separate metal layers for making conductive interconnections between transistors, and more metal layers in the future are likely. This type of multi-layer structure is important in modern IC devices such as field programmable gate arrays, high density memory chips, microprocessors, and others in order to save chip real estate. The active devices are built upwards in many layers forming transistors, capacitors and other components.
When an IC device is found defective during product qualification, quality control testing, or while in service, various failure analysis techniques can be used to determine the cause of the failure. One of these methods utilizes liquid crystal""s chemical response to heat for the identification of the failure site(s). This method is particularly useful for identifying short circuits between metal layers which generate heat in localized areas on a device. This method is also useful for locating leakage current problems that generate localized heat on a device.
The liquid crystal method entails coating a failing IC device with liquid-crystal, heating the liquid crystal to a temperature just below its clear/opaque transition temperature (i.e., the temperature above which liquid crystal blocks light in the visual spectrum), and exercising the failing IC device by applying voltage to the pads of the device. The failing device can either be a die on a wafer or a finished package. Wafer probes may be used to apply the voltage signals to a die, or bonding wires on a previously packaged device may be contacted to apply voltage signals to the device. A packaged device must be decapped to allow coating of the liquid crystal and observance of the device during the test.
When voltage is applied to the device, a short circuit or current leakage problem will cause localized heat generation. Because the liquid crystal is heated to just below its clear/opaque transition temperature, the localized elevated temperatures caused by the failure(s) increase the temperature of the liquid crystal in contact with the affected region above the transition temperature and cause it to change phases locally and become opaque. This clear/opaque transition can be observed by a failure analyst via an optical microscope or other viewing equipment used in a closed circuit computerized system with a monitor. Then, the region affected by the fault can be identified for further testing.
Although this liquid crystal hot spot method is cost effective and efficient at finding faults in IC chips, modern manufacturing processes are making the method less effective. As more layers of conductive and non-conductive materials are added into the semiconductor manufacturing process, the standard liquid crystal hot spot method described above is less able to find faults in lower layers of the IC device. This is because more material lies between the heat generating faults and the liquid crystal. As a consequence, it would be helpful to enhance the standard liquid crystal hot spot method by making liquid crystals more sensitive to heat caused by short circuits or leakage faults in the lower layers of an IC chip.
The present invention solves the above-described problems and provides a distinct advance in the art of integrated circuit chip failure analysis. More particularly, the present invention provides a chemical mixture of liquid crystal and a substance that lowers the clear/opaque transition temperature of the liquid crystal, thins the liquid crystal, and makes the liquid crystal more sensitive to heat generated in the lower layers of an IC chip.
The composition of the present invention broadly includes liquid crystal combined with a substance which lowers its clear/opaque transition temperature and makes it more sensitive to heat generated at lower levels of an integrated circuit chip. While the substance may be acting as a solvent or a diluent, it will be referred to throughout as a solvent. In one embodiment of the present invention, the substance is a ketone. In another embodiment of the invention the substance is an alcohol. In one separate aspect of the invention the ketone or alcohol can be mixed with a liquid crystal wherein the ketone or alcohol is less than approximately 50% by volume of the mixture. In another separate aspect of the present invention, the ketone or alcohol can be mixed with a liquid crystal wherein the ketone or alcohol is approximately 25-35% by volume of the mixture. In one embodiment, the substance is premixed prior to being applied to the device under test. In another embodiment the substance is mixed directly on the device under test.
By utilizing a mixture as described herein, numerous advantages are realized. For example, a mixture of liquid crystal and one of the substances disclosed herein reduces the clear/opaque transition temperature of the liquid crystal and thins the liquid crystal, making it more sensitive to heat generated in the lower layers of an IC chip. This greater sensitivity allows for better failure analysis of IC chips manufactured using modern processes.
These and other important aspects of the present invention are described more fully in the detailed description below.